Since I mentioned Tragedy in Tragidor above (as a starter mod) I guess I will feel free to spin out a few more gems from my historic memory.
A number of newer players may be unfamiliar with these gems that were in their day quite highly thought of.
I would consider these mods to have high re-playability factors as well but. (writing/NPC/immersion)...the downfall of these gems is that they are short / limited / lower level and probably need you to discover some more mods (like above) to further develop the adventurer you want... I cannot begin to tell you how many level 3 - 7 local vault builds still litter my NWN file folder. At least I have hundreds of choices and a spread sheet....
1) Tragedy in Tragidor.
Poignant story telling albeit linear plot. Level 1 - 7 ish as I recall. Some critic on spawns in one section but really be prepared. A Drow companion with an interesting story as I recall.
2) < Insert Name > Murder Mystery Module and others by AUTHOR EL DANO
A bit of an original concept in NWN at the time. High high replay-ability as the the who done it is random if I recall correctly. It changes if you restart the module. This is Neverwinter Nights meets Agatha Christie. Note not much "action" - more a thinking / reading persons cup of tea. El Dano's first module based on a D&D solo book adventure is also very strong
3) UK The Sentinel by (insert Author>
A bit of a rural Cormyrean Nights. NPCs well written and a plot well translated from PnP to NWN. Really wonderful frankly and well rounded NPCs... just short.